A particle moves in a circle of radius $0.5 \mathrm{m}$ at a speed that uniformly increases. Find the angular acceleration of particle if its speed changes from $2 \raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.$ to $4 \raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$s$}\right.$ in $4 \mathrm{seconds}$?